Conventionally, the following various bonding methods have been commonly used for packaging, on a wiring substrate, chip-type LED elements (hereinafter, referred to as LED elements) which are one example of chip-type electronic parts.
The first bonding method is a method using silver paste. As illustrated in FIG. 5, drops of silver paste 53 are applied onto portions of a metal wiring pattern 52 that are placed opposing to each other with each element-mounting section of a wiring substrate 51 located in between. Then, external metal electrodes 55, each formed on each end of an LED element 54, are kept in contact with the drops of silver paste 53, and the metal wiring pattern 52 and the external metal electrodes 55 are bonded by thermosetting the silver paste 53.
The second bonding method is a method using soldering paste. As illustrated in FIG. 6, drops of soldering paste 56 are applied onto the metal wiring pattern 52 of the wiring substrate 51. Then, the external metal electrodes 55 of the LED element 54 are respectively kept in contact with the drops of soldering paste 56, and the metal wiring pattern 52 and the external metal electrodes 55 are bonded by reflow-setting the soldering paste 56.
The third bonding method is a method using anisotropic conductive paste. As illustrated in FIG. 7, anisotropic conductive paste 57 is applied onto the metal wiring pattern 52 of the wiring substrate 51. Then, the external metal electrodes 55 of the LED element 54 are respectively kept in contact with the anisotropic conductive paste 57, and the metal wiring pattern 52 and the external metal electrodes 55 are bonded by setting the anisotropic conductive paste 57 through thermocompression bonding.
In the first and second bonding methods, however, extra portions of the silver paste 53 or the soldering paste 56 tend to leak out; this tend to cause a short circuit between the external metal electrodes 55 or between the portions of the metal wiring pattern 52. This also causes a difficulty in narrowing the gap between the opposing external metal electrodes 55, and thus causes a difficulty in further miniaturizing the LED element 54. For the same reason, it is difficult to narrow the mounting pitch between the LED elements 54 in the case of packaging a number of LED elements 54 on one sheet of the wiring substrate 51, thereby causing a problem in achieving a high-density packaging process. In addition, these bonding methods tend to cause degradation in reliability since the LED element 54 is subjected to thermal stress during the bonding process.
The third bonding method using the anisotropic conductive paste 57, on the other hand, does not cause any short circuit, which is different from the cases wherein the first and second bonding methods are adopted, and makes it possible to achieve a high-density packaging process. In this bonding method, however, it is necessary to conduct the thermocompression bonding while the LED element is kept in a uniform heating and pressure-applying state with respect to the wiring substrate 51, with the anisotropic conductive paste 57 sandwiched in between. For this reason, this method requires a special thermocompression bonding jig (not shown) having a complicated structure, and the bonding process thus requires troublesome and time-consuming jobs. Further, stress, which is caused by thermo-compression bonding during the bonding process, is inevitably applied onto the LED element 54 and the wiring substrate 51. Moreover, it is necessary to increase the size of the external metal electrodes 55 in order to suppress offsets in the height of the LED element 54 during the bonding process and to maintain sufficient contact areas for the anisotropic conductive paste 57.